Remote Induction Sounding of Hanna Underground Coal Burn II, Phase 1, Employing Pseudonoise/ Cross-Correlation Techniques

An underground coal gasification site near Hanna, WY, was remotely sounded with an induction system at the surface one year after the burn. An estimated 1800 tons (40 000 cu. ft.) of coal 275 ft. deep had been converted to low-Btu gas. The wideband loop-loop system employed pseudonoise signals at the transmitter and cross-correlation over 0-50 kHz at the receiver to obtain time signatures in the field. These time-domain impulse responses were converted to frequency responses on a Fourier analyzer system for ease of interpretation. Single-frequency responses at approximately 1 and 5 kHz were extracted from these wideband responses for further investigation. Skindepth calculations showed these to be effective for probing at these depths. Fou in-line traverses conducted over the Hanna II, Phase 1 Site produced strong anomalies of at least 8-to-1 at approximately 5 kHz. Interpretation of these anomalies agrees quite well in location and shape with chemical modeling estimates from north to south. However, these soundings support the conclusion that the burn may have protruded another 5 m to the west from the chemical estimate. The centroid of the burn region was obtained by averaging anomaly values weighted by their location.